Dragonkin get the racial any time they land a blow on an opponent that is below 50% after the blow. The only limitation is you don't get the buff it is currently up so you can get it every other turn basically. A little unclear with the description but its always worked this way.

It's not new. Killing your opponent essentially is getting your opponent below 50% Health. This is very nice if you can plan around it, which can be difficult in PvP.

For example against Darkmoon Faire trainer I use a Infinite Whelpling against the Eye.Turn:1. Tail Sweep.2. Tail Sweep. Eye will now be below 50% so dragonkin racial kicks in. This will be overkill so in turn I won't attack.3. Heal.4. Tail Sweep. Kill the Eye and dragonkin racial kicks in again.5. Early Advantage with Dragonkin Racial will pretty much kill off the Darkmoon Tonk.

This is just an example. But if you can plan around it you will really improve your damage output.

You get the buff anytime a hit leaves the opponent below 50% (dead counts as below 50%) and you don't currently have it up already. They could already be below 50% and your hit doesn't kill them off and if you don't have the buff up you'll get it.